When a sample solution containing a sample component separated by a column of a liquid chromatograph is analyzed by a mass spectrometer, an atmospheric pressure ionization method (API), such as an electrospray ionization method (ESI) and an atmospheric pressure chemical ionization method (APCI), is generally used (see Patent Document 1, for example). FIG. 6(A) is a diagram showing the principle configuration of the ESL and FIG. 6(B) is a diagram showing the principle configuration of the APCI.
In the ESL the sample solution is introduced into a thin nozzle 1 having a tip to which a direct-current high-voltage of about several kilovolts is applied. The high voltage applied from a direct-current voltage source 3 imparts, to the sample solution, an electric charge having the same polarity as that of the high voltage. An electric field formed due to the potential difference between the nozzle 1 and an ion-introducing device 4 (for example, a sampling cone or a desolvation tube) arranged opposite to the nozzle 1 acts on the sample solution so that the sample solution is atomized from the tip of the nozzle 1 as it is torn apart from the nozzle 1. In order to assist the atomization of the sample solution, nebulizer gas is used which is blown out from a nebulizer gas tube 2 forming an external cylinder coaxial with the nozzle 1. The charged minute droplet sprayed out from the nozzle 1 is broken apart and micronized due to a collision with surrounding atmospheric gas and electrostatic repulsion occurring inside the minute droplet. Further, the environment is heated to an appropriate temperature, so that solvent in the minute droplet is vaporized. In a process of the micronization of the droplets, an ion derived from the sample component is released therefrom, and the generated ion is conveyed to a non-illustrated mass spectrometric device via the ion-introducing device 4.
Meanwhile, in the APCI, the sample solution is sprayed from the tip of the nozzle 1 using the nebulizer gas (such as nitrogen) brown out through the nebulizer gas tube 2. The solvent in the droplets constituting a spray flow is rapidly vaporized in the atmosphere heated to a high temperature by a heater 5. A discharge electrode 6 is arranged ahead of the spray flow. Due to corona discharge generated by the discharge electrode 6, a solvent molecule is ionized and becomes a reactant ion. The reactant ion and a sample molecule react chemically, causing the sample molecule to become an ion upon the proton addition or proton elimination, followed by being sent to the non-illustrated mass spectrometric device via the ion-introducing device 4.
As mentioned above, in the atmospheric pressure ionization method, in order to ionize the sample component in each of the sprayed droplets, a method is adopted to accelerate the vaporization of the solvent in each of the droplets by spraying high-temperature assist gas or applying heat by a heater. However, in such a conventionally adopted heating method, any drying processes cannot be performed but only a drying process by the ambient temperature at several hundred degrees Celsius at the highest. This is not necessarily enough to vaporize the solvent in a short time.
Therefore, for example, a technique has been devised in which a capillary tube having a small diameter and heated at a high temperature is used as the ion-introducing device 4 to promote the desolvation inside the capillary tube so as to enhance the production of ions (see Patent Document 2, for example). However, it is unavoidable that sample components in the droplets that do not enter the heated capillary tube will be wasted. Particularly, in the ESI, since the size of each of the droplets sprayed from the nozzle 1 is relatively large, it is not easy for the solvent in each of the droplets to vaporize, causing the ratio of the wasted sample components to be large. In order to obtain higher sensitivity in a mass spectrometer using the atmospheric pressure ionization method, it is important to allow the sample components which have conventionally been wasted to be surely ionized, and to supply them to the mass spectrometry.
Meanwhile, for the ionization of a component in a solid-state (or a caked-state) sample, a laser desorption ionization method (LDI) including, as a representative thereof, a matrix-assisted laser desorption ionization method (MALDI), is frequently used. In addition, an atmospheric pressure MALDI (AP-MALDI) is well known in which the MALDI is applied in an atmosphere at approximately atmospheric pressure. However, there is a problem that it is difficult to enhance ionization efficiency in the case of the LDI without using a matrix. In the case of the MALDI also, there are some problems, such that a matrix having an adequate laser beam absorbing quality must be chosen, and a large difference in the sensitivity may arise depending on the mixing state of the sample crystal and the matrix. In view of the above issues, development of a new ionization method instead of the LDI has been desired.